Extrusion die assembly for high density products

ABSTRACT

High-capacity extrusion die assemblies ( 20, 90, 130, 140, 180, 252 ) each having a tubular sections ( 44, 146, 162, 268 ) and an elongated, axially rotatable, helically flighted screw section ( 56, 56   a   , 152, 168, 276, 278 ) which cooperatively define frustoconical, outwardly diverging material flow paths ( 75, 160, 291 ) at constant or differing divergence angles of from about 1-11°. The use of diverging tubular sections ( 44, 146, 162, 268 ) and screw sections ( 56, 56   a,    152, 168, 276, 278 ) permits the use of larger die plates ( 76, 118, 292 ) with an increased number of die openings ( 80, 124, 296 ). This allows significant increases in extrusion production rates. The die assemblies ( 20, 90, 130, 140, 180, 252 ) can be used in the production of a wide number of human foods or animal feeds, and particularly aquatic feeds of the floating or sinking variety. In another aspect of the invention, an extruder ( 210 ) is provided having diverging and converging sections ( 212, 214 ) along the length thereof and defining corresponding flow paths ( 230, 246 ).

CROSS REFERENCE TO RELATED APPLICATION

This is a division of identically titled application Ser. No.12/420,725, filed Apr. 8, 2009, incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with improved, high capacitydie assemblies for use with single or twin extruders, and which arespecially designed for the production of higher density products such assinking aquatic feeds. More particularly, the invention is concernedwith such die assemblies, extruders including the die assemblies, andcorresponding methods wherein the die assemblies preferably include aplurality of elongated, obliquely oriented extension tubes operablycoupled with an extruder barrel, and having a die plate at the outer endof the ends of the extension tubes. Use of the oblique tubes allows useof a larger area die plate having a greater number of die openingstherethrough to thus increase extrusion rates.

2. Description of the Prior Art

Extrusion processing of comestible products such as human foods andanimal feeds has long been practiced and is a highly developed art. Ingeneral terms, food extruders of the single or twin screw variety areemployed, having elongated, tubular barrels with inputs adjacent one endthereof and restricted orifice dies at the outlet thereof, and one ortwo helically flighted, rotatable screws within the barrel. In manyinstances steam is injected into the barrel during processing. Anapertured die plate is operably coupled with the outlet end of thebarrel in order to form the product as it emerges from the extruder.Depending upon the selected extrusion conditions, the final products maybe fully or partially cooked, and can have varying degrees of expansion.

Many commercial extruders are designed with converging terminal screwsections sometimes referred to as “cone nose” sections. These screwsections are housed within a complementally converging barrel section.These converging extruder end assemblies are provided in order toincrease the pressure and shear conditions within the extruder justupstream of the final extrusion die. Typical examples of these designsare found in U.S. Pat. Nos. 4,118,164 (single screw) and 4,875,847 (twinscrew).

Referring to FIG. 18, conventional prior art food extrusion assembliesare generally made up of a serially interconnected preconditioner 10 andextruder 11, the latter having a multiple section barrel 12 terminatingin a restricted orifice die 13, and one or two internal, helicallyflighted, axially rotatable screw(s) 14. The screw 14 include anelongated central shaft 15 with helical fighting 16 along the lengththereof. In many cases, the terminal head and screw sections 17 and 18are of converging, frustoconical design in order to increase thepressure and shear conditions within the barrel just upstream of die 13.

In operation, a comestible material (usually a mixture of ingredientsincluding quantities of protein, starch and fat) is fed intopreconditioner 12 where it is initially moisturized by the addition ofsteam and/or water and heated to partially cook the material. Thepreconditioned material is then delivered to the extruder 11 where theaction of the rotating screws(s) 14 serves to convey the material towardand through die 13. During this conveyance, the material is subjected toincreasing levels of temperature, pressure and shear, in order to cookthe material to the desired degree. As the material emerges from the die13, it is formed as a final product and may undergo expansion as aresult of flash-off of moisture from the material. The degree ofexpansion is a controlled phenomenon, and is influenced by the amount ofenergy imparted to the material within the barrel 12 and the geometry ofthe final die 13.

These food extruder systems have been used for decades to produce a widevariety of human foods and animal feeds. However, it has been found thatthe frustoconical end configuration of the terminal barrel and screwsections 17 and 18 may limit the rate of production achievable with suchextrusion assemblies. Specifically, owing to the fact that the terminalbarrel and screw sections converge, the die 13 necessarily has a reducedsurface area, and hence can only have a certain number of restricted dieopenings therein. Indeed, this limitation on the number of available dieopenings is the limiting factor in production rates for some productssuch as small diameter or micro-aquatic feeds.

For example, using a typical Wenger Model X165 single screw extruder forthe production of micro-aquatic feeds having a diameter of up to 2 mm.,the maximum throughput is on the order of 1-1.5 tons/hr., and this ratelimitation is attributable to the presence of only a small number of dieholes.

Other conventional extruder designs are disclosed in U.S. Pat. Nos.3,728,053; 3,904,341; 4,346,652; 4,352,650; 4,400,218; 4,422,839;4,836,460; 5,458,836; 6,074,084; 6,331,069; 6,491,510; 7,101,166;Japanese Patent No. JP 55013147; Non-Patent Literature: Effects of DieDimensions on Extruder Performance; Sokhey A S; Ali Y; Hanna M A;Foodline; Influence of Extrusion Conditions on Extrusion Speed,Temperature, and Pressure in the Extruder and on Pasta Quality;Abecassis, J.; Abbou, R; Food Sci.&Tech. Abs; and Barrel-Valve Assembly:Its Influence of Residence Time Distribution and Flow Pattern in aTwin-screw Extruder; Liang, M.; Hsieh, F; AGRICOLA.

SUMMARY OF THE INVENTION

The present invention overcomes the problems outlined above, andprovides high capacity extrusion die assemblies for use with single ortwin screw extruders. Generally speaking, the die assemblies include anelongated tubular section having an axial length, a smaller diameterinlet end and a larger diameter outlet end, with an internal boreprogressively diverging at an angle of from about 1-11° (more preferablyfrom about 1.5-7° and most preferably from about 2-4°) in a directionfrom the inlet end towards the outlet end. An elongated, axiallyrotatable screw section is located within the tubular section and has anaxial screw length with a smaller diameter inlet end proximal to thetubular section inlet end and a larger diameter outlet end proximal tothe tubular section outlet end. The screw section includes an elongatedshaft with outwardly extending helical fighting presenting fightingouter surfaces along the length of the shaft, the fighting outersurfaces progressively diverging at an angle of from about 1-11° (morepreferably from about 1.5-7° and most preferably from about 2-4°) in adirection from the inlet end toward the outlet end. A die unitassociated with the tubular section outlet end and has a plurality ofdie openings therethrough configured to create a pressure drop acrossthe die openings during extrusion operations.

Thus, in preferred forms the mating tubular and screw sections define anoutwardly diverging flow path having the aforementioned divergence anglewhich is generally frustoconical in overall shape. Consequently, the dieunit can have a larger overall area as compared with the prior art, sothat more die openings can be provided to increase extruder capacity.

Although the tubular and screw sections are preferably formed withgradually and progressively diverging defining surfaces throughout theentire axial lengths thereof, in certain instances the divergence neednot be throughout the entirety of these sections. More generally, thetubular and screw sections should have diverging surfaces over at leastabout 50% of the lengths thereof, and more preferably at least about 75%of the lengths thereof. Additionally, while in certain embodiments thedivergence angles of the tubular and screw sections are the same, inother cases these angles may be different. Also, respective portions ofthe tubular and screw sections may be of different divergence angles.For example, a given portion of the die assembly may have a relativelysmall divergence angle while a downstream portion of the die assemblymay be configured with a larger divergence angle.

In preferred forms, the tubular section of the die assembly is made upof a cylindrical body with a replaceable internal sleeve having adesired diverging surface, so that the body and sleeve cooperativelydefine the overall head section. In other cases, however, a unitarytubular section can be used.

Where the die assemblies of the invention are used for the production oflow density products such as floating micro-aquatic feeds, the die unitcomprises a die plate which is located in close proximity to the end ofthe screw section. This is to avoid a situation where the material beingprocessed is allowed to densify prior to passage through the die plate.

On the other hand, where the die assemblies are employed in theproduction of high density products such as sinking aquatic feeds, thedie plate is spaced a significant distance from the terminal end of thescrew section. Advantageously, in such dies a plurality of obliquelyoriented, structurally distinct and separate material flow tubes arepositioned at the outlet of the screw section, and a die plate ismounted at the other end of the flow tubes. Indeed, with the use of suchoblique flow tubes, adequate divergence can be obtained without the useof the upstream diverging tubular and screw sections, or for that mattereven without the use of a final screw section. Thus, such high productdensity die assemblies preferably include an elongated tubular sectionpresenting a longitudinal axis and having an axial length and a borewith an inlet end and an outlet end, with a manifold operably coupledwith the tubular section outlet end in order to receive material fromthe outlet end. A plurality of tubular extensions are secured to amanifold and are configured to receive material from the manifold, eachof the extensions being oriented at an oblique angle relative to thelongitudinal axis of the tubular section. A die is operably coupled witheach of the extensions adjacent the ends thereof remote from themanifold and including a plurality of die openings oriented to create apressure drop across the die openings during extrusion operations. Eachof the extensions is independently oriented at an angle of from about2-12°, more preferably from about 4-10° relative to the longitudinalaxis of the tubular section.

In preferred practice however, the upstream tubular section adjacent themanifold is equipped with an axially rotatable screw section, and theupstream tubular and screw sections are as described above, i.e., thesehave a diverging construction to present a generally frustoconical flowpath through these sections.

The die assemblies create methods of extruding materials from anextruder barrel comprising the steps of moving material under pressurealong a progressively diverging, generally frustoconical path of traveldefined between a stationary tubular section having an axial length withan internal bore presenting a smaller diameter inlet end and a largerdiameter outlet end, and an axially rotating, helically flighted screwsection within the bore. The path of divergence corresponds with thedivergence angle of the tubular and screw sections, and is generallyfrom about 1-11° (more preferably from about 1.5-7° and most preferablyfrom about 2-4°) in a direction from the inlet end toward the outletend. The path of travel may progressively diverge over substantially theentirety of the bore axial length, but in any event should progressivelydiverge over at least about 50% of such axial length. After travelingalong the flow path the material passes through a plurality ofrestricted orifice die openings so as to create a pressure drop acrossthe die openings.

As noted previously, where the die assemblies are designed forproduction of low density products, the die is closely adjacent theterminal end of the screw section. Where high density products aredesired and use is made of the diverging flow tubes, the material isforced through these tubes by the action of the extruder in order toallow the material to cool and densify in the tubes prior to extrusion.

In another aspect of the invention, improved extruders are providedhaving alternating diverging and converging sections along the lengththereof. Such an extruder includes an elongated tubular barrelpresenting an inlet end and an outlet end, and an elongated, axiallyrotatable, helically flighted screw within the barrel and operable tomove the material from the inlet toward and through the outlet underpressure. A die assembly is coupled to the outlet end of the barrel andpresents a plurality of die holes therethrough configured to create apressure drop across the die openings during passage of the materialthrough the openings. The extruder barrel has first and second sectionsalong the length thereof, wherein the first section having a firstgenerally frustoconical bore with the large end thereof proximal to theinlet end and the small end thereof proximal to the outlet end. Thesecond section has a second generally frustoconical bore with the largeend thereof proximal to the outlet end and the small end thereofproximal to the inlet end. The screw assembly also has first and secondsections along the length thereof and correspondingly received withinthe first and second tubular sections. The first screw section is ofgenerally frustoconical configuration and in alignment with the firsttubular section, and the second screw section of generally frustoconicalconfiguration in alignment with the second tubular section. Accordingly,the first tubular section and the first screw section cooperativelydefining a converging material flow path along the length thereof andtowards the outlet end, and the second tubular section and the secondscrew section cooperatively defining a diverging material flow pathalong the length thereof and towards the outlet end. The alternatingconverging and diverging extruder sections may be spaced apart along thelength of the barrel, but are preferably adjacent one another.

A conventional die plate may be used with the extruders of theinvention. However, to obtain the maximum benefit of the invention, theimproved die assemblies hereof are used with the extruders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a die assembly in accordance with theinvention for the production of low density extrudates, coupled to theoutlet end of an extruder barrel;

FIG. 2 is an end view of the assembly illustrated in FIG. 1;

FIG. 3 is a side view with parts broken away of the assembly illustratedin FIG. 1;

FIG. 4 is a vertical sectional view of the assembly of FIG. 1;

FIG. 5 is a perspective view of another die assembly in accordance withthe invention for the production of high density extrudates, coupled tothe outlet end of an extruder barrel and depicting a cut-off knifeadjacent the die face;

FIG. 6 is an end view of the assembly illustrated in FIG. 5;

FIG. 7 is a side view with parts broken away of the assembly illustratedin FIG. 5;

FIG. 8 is a vertical sectional view of the assembly of FIG. 5;

FIG. 9 is a perspective view of the die assembly of FIG. 5, shownseparate from an extruder barrel and without a cut-off knife;

FIG. 10 is an end view of the die assembly of FIG. 9;

FIG. 11 is a perspective view of the die assembly of FIG. 9, from theopposite end thereof as illustrated in FIG. 9;

FIG. 12 is an end view of the FIG. 9 die assembly, illustrating the endopposite that shown in FIG. 10;

FIG. 13 is a vertical sectional view of another die assembly mounted onthe end of an extruder barrel, and having a variable depth, decreasingvolume screw as a part of the die assembly;

FIG. 14 is a vertical sectional view of another die assembly having apair of aligned, diverging screw sections, wherein the terminal screwsection adjacent the die has a greater angle of divergence than theadjacent screw section;

FIG. 15 is a vertical sectional view of an extruder and die assemblywherein the terminal screw section of the die assembly is straight andthe adjacent screw section is of diverging configuration;

FIG. 16 is a vertical sectional view of an extruder and die assemblywherein the terminal screw section of the die assembly is of divergingconfiguration and the adjacent screw section is of convergingconfiguration;

FIG. 17 is a vertical sectional view of a twin screw extruder equippedwith a die assembly in accordance with the invention, wherein the screwsection includes a pair of intercalated screw sections;

FIG. 18 is a vertical sectional view of a typical prior artpreconditioner/extruder assembly;

FIG. 19 is a fragmentary, vertical sectional view of a single screwextruder including the a die assembly of the type depicted in FIGS.5-12, with a back pressure valve interposed between the outlet end ofthe extruder and the inlet end of the die assembly;

FIG. 20 is a fragmentary, vertical sectional view of a twin screwextruder of the type illustrated in FIG. 17, with the die assembly ofFIGS. 5-12; and

FIG. 21 is a fragmentary, vertical sectional view of the twin screwextruder of FIG. 20, with a die of the type illustrated in FIGS. 1-4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment of FIGS. 1-4Extrusion Die Assembly for the Production of Low Density Products

Turning now to FIGS. 1-4, a die assembly 20 is illustrated, attached tothe end of a conventional single screw extruder not having a convergingend section. The extruder is made up of a plurality of tubular,end-to-end interconnected head sections defining a stationary tubularbarrel having an inlet and terminating in a final outlet head section24. An elongated, multiple-section, helically flighted, axiallyrotatable screw assembly is positioned within the barrel and terminatesin a final screw section 30 within final head section 24; the screwassembly is operable to move material from the inlet end of the barreltoward and through the outlet end thereof. The screw sections eachinclude a central shaft 32 and a plurality of outwardly extendinghelical flights 34. The shafts 32 are in turn mounted on a central driveshaft 36 which is coupled with a rear motor and drive assembly (notshown), for rotation of the shaft 36 without translatory movementthereof, best seen in FIG. 4, the head sections making up the extruderbarrel may be equipped with internal, straight or helically ribbedsleeves 38 cooperatively defining an elongated bore 39 along the lengthof the barrel. The tubular sections may have ports 40 for injection ofsteam and/or water into the barrel 26. Additionally, these sections mayhave ported external jackets 42 to allow indirect heat exchange controlof the temperature within the barrel through use of water or steaminjection.

The die assembly 20 is mounted on final head 24 and includes a tubularsection 44 having a heat exchange jacket 46. The tubular section 44 alsohas an internal, helically ribbed sleeve 48 which defines an internalfrustoconical bore 50 in communication with bore 39. It will be observedthat the bore 50 has an inlet end 52 of relatively small diameter and anoutlet end 54 of relatively large diameter, and a gradually andprogressively diverging bore-defining wall surface 55 extending frominlet end 52 to outlet end 54. The length/diameter (L/D) ratio oftubular section 44, namely the ratio of the axial length of the tubularsection 44 divided by the inner diameter of the small inlet end 52, isabout 2.03. In general, with the die assemblies of this embodiment theL/D ratio should be from about 1.5-4, more preferably from about 2-3.

The die assembly 20 also has a screw section 56 within tubular section44 and secured to a reduced diameter drive shaft extension 58 by meansof bolt 60. The screw section 56 includes a shaft 62 and outwardlyextending helical fighting 64. The screw section 56 also presents aninlet end 66 adjacent final screw section 30 and an opposed outlet end68. The shaft 62 has an outer defining surface 70 which diverges frominlet end 66 to outlet end 68. Similarly, the fighting 64 presentsfighting outer surfaces 72 which also diverge in the same fashion assurface 70. In this embodiment, the surfaces 55, 70 and 72 diverge at aconstant angle of 3°, and cooperatively define a generally frustoconicalflow path 75 from the inlet ends 52 and 66 to the outlet ends 54 and 68.In practice, divergence angles of from about 1.5-7°, and more preferablyfrom about 2-6°, have been found to be suitable.

The die assembly 20 also has a die unit 74 in the form of a die plate 76secured to tubular section 44 by means of bolts 78. The die plate 76 hasa plurality of restricted orifice die openings 80 arranged in agenerally circular pattern so as to be in direct communication with theoutlet end of flow path 75, and to create a pressure drop across theopenings 80 during extrusion. The die plate 76 is also equipped with acentral stub shaft 82 to facilitate mounting of a rotatable knife (notshown) adjacent the outlet face of the die plate 76. In this embodiment,the die plate 76 is closely adjacent the outlet end 68 of screw section56 which is significant in the production of low density extrudates.Generally, the die plate 76 should be positioned at a distance of fromabout 0.5-2 inches from the outlet end 68, or more generally at adistance less than the diameter of die plate 76.

In the production of low density extrudates using die assembly 20, astarting mixture is passed into and through the extruder barrel andultimately through die assembly 20. During such passage, the screwassembly and die screw section 56 are rotated by shaft 36. If desired,steam and/or water may be injected into the barrel 26 via ports 40, andfurther temperature control may be achieved by passing cold water orsteam through the jackets 42. This has the effect of subjecting thestarting mixture to increasing levels of temperature, pressure and shearin order to cook the mixture to the desired extent. The material passingfrom barrel 26 is conveyed along the diverging, generally frustoconicalflow path 75 towards and through the die openings 80. The extrusionparameters (e.g., SME, STE, and extruder configuration) are maintainedso as to impart significant energy to the material in order to increasethe degree of expansion of the product as it emerges from the dieopenings 80.

A variety of low density extrudates can be produced using the dieassembly 20. A prime example of such a product is floating fish feedsdesigned to largely float at or near the surface of water. In theproduction of such feeds, the starting mixtures would typically containone or more grains (e.g., corn, wheat, oat, milo, soy) as well asproteinaceous materials such as fish meal. The starting mixture wouldnormally be initially processed and moisturized in a preconditioner suchas a Wenger DDC preconditioner or a preconditioner as disclosed in U.S.Pat. No. 7,448,795. In the extruder, the following conditions aretypical: residence time of the material being processed within theextruder barrel of from about 3-20 seconds, more preferably from about4-10 seconds; extruder screw speeds of from about 250-900 rpm, morepreferably from about 400-800 rpm; maximum temperature of material beingprocessed within the barrel, 100-150° C., more preferably from about110-125° C.; maximum pressure within the barrel of from about 100-2000psi, more preferably from about 400-800 psi. In such processing thematerial may be cooked (as measured by extent of gelatinization ofstarch-bearing ingredients) to any desired level, but usually cooklevels of at least about 75%, more preferably from about 75-98% areachieved. The floating fish feeds usually contain from about 18-36% byweight protein, from about 2-5% by weight fat, and from about 20-50% byweight starch, and have as-extruded moisture levels of from about 21-23%by weight, and wet product densities of from about 410-460 g/l.

It has been found that use of die assembly 20 overcomes the limitationon production experienced with conventional extrusions systems, in thatthe die plate 76 has a much larger number of die openings 80 as comparedwith the typical smaller die plates 13. Indeed, production rates arevery significantly enhanced, often by a factor of three or more, withoutany loss in product quality.

Embodiment of FIGS. 5-12 Extrusion Die Assembly for the Production ofHigh Density Products

FIG. 5 illustrates a die assembly 90 operably coupled with the finalhead section 24 of an extruder, as in the case of the first embodiment.The die assembly 90 includes a tubular section 44 and screw section 56,again as set forth in the first embodiment. Inasmuch as the sections 24and 44, and the screw sections 30 and 56, are identical with the earlierembodiment, like reference numerals have been used in FIGS. 5-12 and nofurther discussion of these components is required.

The overall die assembly 90 has, in addition to the previously notedtubular section 44 and screw section 56, a manifold 92, obliqueextension tubes 94, and a final die unit 96. The manifold 92 is securedto the butt end of tubular section 44 by means of bolts 98 and includesa main body 100 having a rear wall 102, forwardly extending annular wall104 and outer wall 106. As best seen in FIGS. 11 and 12, the rear wall102 has a central opening 108 which communicates with frustoconical flowpath 75. A somewhat Y-shaped divider 110 extends between the walls 102and 106 and serves to divide the opening 108 into three equal areasections 108 a, 108 b, 108 c. The outer wall 106 has three equidistantlyspaced, circularly arranged openings 112 which respectively communicatewith the sections 108 a, 108 b, 108 c. An extension tube 94 is securedwithin each opening 112 which serves to orient the tubes at an obliqueangle relative to the central longitudinal axis of the body 100. In theillustrated design, each of the tubes 94 is oriented at approximately a6° angle of divergence from this axis; more generally a divergence angleof from about 2-12° and more preferably from about 4-10°, is suitable.The tubes 94 may be of variable length, but generally have an axiallength of from about 6-24 inches.

The die unit 96 is secured to the tubes 94 adjacent the ends thereofremote from body 100. In particular, the die unit 96 includes a mainbody 114 having three equally circumferentially spaced, frustoconicalbores 116 therethrough. As best seen in FIG. 8, the outboard ends of thetubes 94 are respectively received within the bores 116. A die plate 118is secured to the outer face of main body 114 by bolts 120. The dieplate 118 has three die sections 122 respectively covering the outerends of the bores 116. Each of the die sections 122 has a series ofrestricted die openings 124 therethrough, which create a pressure dropacross the openings during extrusion. A mounting stub shaft 126 extendsthrough plate 118 and is secured within body 114. The stub shaft 126permits mounting of a rotatable cutting knife 128.

In preferred forms, the tubes 94 are circular in cross section and arearranged in a substantially circular array. However, the invention isnot so limited and the tubes can have different cross sectional shapesand can be arrayed differently. Where the term “diameter” is used withrespect to the tubes 94, this is intended to denote the largest crosssection dimension where non-circular tubes are employed. In addition, itis preferred that the tubes 94 be static, i.e., they do not include anymoveable components along the lengths thereof, and have a constant shapeand diameter along these lengths. Preferably, the length/diameter (L/D)ratio for the tubes 94 is from about 2-8, more preferably about 3-5, andmost preferably about 3.75. It is also preferred that the tubes beseparate and out of communication with each other at the ends thereofadjacent the outer apertured die sections 122. This is to be contrastedwith a situation where separate flow paths are provided in which thematerial from the respective flow paths is recombined prior to extrusionthereof.

During extrusion using die assembly 90, the preconditioned material fedinto the extruder barrel is subjected to increasing levels of shear,temperature, and pressure, and passes from the extruder barrel intomanifold 92 where the material is separated into three streams owing tothe presence of the diverter 110. These individual streams are thenforced through the separate, structurally distinct tubes 94 towards andthrough die sections 122 of die plate 118. As such, the material passesthrough fully separate, mutually diverging paths of travel. During thetravel through the tubes 94, the material is densified and cooled sothat the final extrudates have a relatively high density. Moreover, thetubes 94 result in longer retention times and thus more cook for theproduct, as compared with conventional dies. The use of the manifold 92and the separate tubes 94 in place of a single, large tube or extensionhaving the same total cross sectional area results in a higher pressuredrop across the dies, thus creating a denser product. Of course, theoperating conditions of the extruder are maintained so that excessexpansion upon extrusion is avoided. In general, the conditionsspecified above for the first embodiment may be followed with dieassembly 90.

A principal use for die assembly 90 is in the production of sinking fishfeeds which are designed to descend in water at controlled rates (e.g.,slow- and fast-sinking feeds) which are optimum for specific species offish. Common slow-sinking fish feeds have protein levels of from about26-45% by weight, fat levels of from about 20-40% by weight, and starchlevels of from about 5-15% by weight. Correspondingly, the denserfast-sinking feeds usually have protein levels of from about 26-48% byweight, fat levels of from about 18-26% by weight, and starch levels offrom about 10-15% by weight. The as-extruded moisture levels of theslow- and fast-sinking fish feeds are from about 18-22% by weight and26-28% by weight, respectively. The wet products have densities of fromabout 510-570 g/l (slow-sinking) and 600-650 g/l (fast-sinking). Anothertype of dense aquatic feed which can be produced by die assembly 90 isshrimp feed, which usually has from about 22-32% by weight protein, 2-6%by weight fat, and 12-26% by weight starch. These products have anas-extruded moisture level of from about 27-31% by weight and a wetproduct density of from about 660-720 g/l.

Embodiment of FIG. 13 Extrusion Die Assembly for the Production of HighDensity Products with a Variable Depth, Decreasing Volume

FIG. 13 illustrates an alternate die assembly 130, again illustrated asmounted on the final bead section 24 of an extruder. The illustratedextruder barrel and screw components are identical to that of FIG. 4,and like reference numerals have therefore been used. The die assembly130 also includes many of the same components as die assembly 20, andidentical components are numbered as in FIG. 4, and similar butdifferently components have the same reference numerals as in FIG. 4except for the provision of an “a” distinguishing identifier. The dieassembly 130 differs from die assembly 20 in that the latter had aconstant free volume along the length thereof, whereas die assembly 130has a variable Righting depth and decreasing free volume along itslength. This is achieved by different relative geometries of the tubularand screw sections.

The die assembly 130 is mounted on final head 24 and includes a tubularsection 44 having a heat exchange jacket 46. The tubular section 44 alsohas an internal, helically ribbed sleeve 48 a which defines an internalfrustoconical bore 50 a in communication with bore 39. It will beobserved that the bore 50 a has an inlet end 52 a of relatively smalldiameter and an outlet end 54 a of relatively large diameter, and agradually and progressively diverging bore-defining wall surface 55 aextending from inlet end 52 a to outlet end 54 a.

The die assembly 130 also has a screw section 56 a within tubularsection 44 and secured to a reduced diameter drive shaft extension 58 bymeans of bolt 60. The screw section 56 a includes a shaft 62 a andoutwardly extending helical fighting 64 a. The screw section 56 a alsopresents an inlet end 66 a adjacent final screw section 30 and anopposed outlet end 68 a. The shaft 62 a has an outer defining surface 70a which diverges from inlet end 66 a to outlet end 68 a. Similarly, thefighting 64 a presents fighting outer surfaces 72 a which also diverge.In this embodiment, the surfaces 55 a and 72 a each diverge at aconstant angle of 3°. However, the fighting depth of screw section 56 adecreases from the inlet end 66 a to the outlet end 68 a thereof.“Flighting depth” refers to the distance between the outer fightingsurfaces 72 a and the defining outer surface of the shaft 62 a. Aconsideration of FIG. 3 will demonstrate that screw section 56 of dieassembly 20 has a constant flight depth, whereas the fighting depth ofscrew section 56 a decreases as explained. In this design, the angle ofdivergence of the surface 70 a is 6°, whereas the angle of divergence ofthe surfaces 72 is 3°. The die assembly 130 also has a die unit in theform of a die plate 76 identical with that of FIG. 3.

The FIG. 13 embodiment may be used in the production of very low densityfloating fish feeds. The decreasing flight depth and free volume withinthe die assembly 130 imparts more energy into the material beingextruded.

Embodiment of FIG. 14 Extrusion Die Assembly Having a Pair ofInterconnected Tubular and Screw Sections with Different Angles ofDivergence

FIG. 14 illustrates a die assembly 140 which includes a pair ofinterconnected sections 142 and 144. Although not shown, the section 142is connected to the terminal section 24 of an extruder barrel. Thesection 142 includes a tubular section 146 similar to tubular section44. The tubular section 146 has an internal, helically ribbed sleeve 148presenting an outer surface 150. The section 142 also has an elongated,helically flighted, axially rotatable screw section 152 therein,including a progressively tapered shaft 154 and outwardly extendinghelical fighting 156 presenting outermost fighting surfaces 158. Thetubular section 146 (owing to the presence of the sleeve 148) and screwsection 152 have corresponding, smaller diameter inlet ends 146 a, 152a, and larger diameter outlet ends 146 b, 152 b. Accordingly, thetubular section 146 and screw section 152 cooperatively define agenerally frustoconical flow path 160 along the length thereof. In theillustrated embodiment, both of the surfaces 150 and 158 are oriented ata constant divergence angle of 3°.

The second or terminal section 144 also has a tubular section 162equipped with an internal, helically ribbed sleeve 164 presenting anouter surface 166. The section 144 also has an elongated, helicallyflighted, axially rotatable screw section 168 therein, including aprogressively tapered shaft 170 and outwardly extending helical fighting172 presenting outermost fighting surfaces 174. The tubular section 162(owing to the presence of sleeve 164) and screw section 168 havecorresponding, smaller diameter inlet ends 162 a, 168 a, and largerdiameter outlet ends 162 b, 168 b. Accordingly, the tubular section 162and screw section 168 cooperatively define a generally frustoconicalflow path 176 along the length thereof. Both of the surfaces 166 and 174are oriented at a constant divergence angle of 6°.

Embodiment of FIG. 15 Extrusion Die Assembly Having a Straight TerminalSection

FIG. 15 depicts a die assembly 180 having all of the components of dieassembly 20, with the addition of a straight terminal section.Accordingly, the components of assembly 180 identical with those of dieassembly 20 are identically numbered.

Additionally, the die assembly 180 includes a straight section 182secured to the outlet end of tubular section 44. The section 182 has atubular section 184 with a ribbed internal sleeve 186 having an innersurface 188. In this instance, the inlet and outlet ends 190, 192 of thetubular section 184 are of the same size, and the surface 188 does notdiverge. The section 182 also has an elongated, helically flighted,axially rotatable screw section 194 therein, having a shaft 196 andhelical fighting 198 presenting fighting outer surfaces 200. It will beseen that the surfaces 188 and 200 are each straight in that they have azero angle of divergence. A die plate 76 identical to that of FIG. 3 ismounted on the outlet end of tubular section 184 as illustrated.

Embodiment of FIG. 16 Extruder and Die Assembly Having AlternatingDiverging and Converging Sections

FIG. 16 illustrates a situation where an extruder 210 is provided havingalternating diverging and converging sections, with a final die assembly20. Again, the die assembly 20 is identical with that described in thefirst embodiment and like reference numerals are used in FIG. 16.However, the upstream extruder sections leading to die assembly 20 havea diverging section 212 and a converging section 214. The divergingsection 212 has a tubular head 216 with an internal, helically ribbedsleeve 218 presenting an inner surface 220. An elongated, helicallyflighted, axially rotatable screw section 222 is located within headsection 216 and has a central shaft 224 with helical fighting 226presenting fighting surfaces 228. The inlet ends 216 a, 222 a and outletends 216 b, 222 b of head section 216 (because of the presence of sleeve218) and screw section 222 are respectively of smaller and largerdiameter, so as to cooperatively define a generally frustoconical anddiverging flow path 230 along the length of section 212.

The converging section 214 has tubular head section 232 with aninternal, helically ribbed sleeve 234 presenting an innermost surface236. An elongated, helically flighted, axially rotatable screw section238 is located within head section 232 and has a central shaft 240 withhelical fighting 242 presenting fighting surfaces 244. In this case,however, the inlet ends 232 a, 238 a of the head section (because of thepresence of sleeve 234) and screw section 232, 238 are larger than thecorresponding output ends 232 b, 238 b. Accordingly, the surfaces 236and 244 cooperatively define a generally frustoconical but convergingflow path 246. The components of sections 212 and 214 are designed sothat the diverging flow path 230 and converging flow path 246 are eachoriented at 3°. As indicated, the die assembly 20 has a diverging 3°flow path 75 as well.

Embodiment of FIG. 17 Die Assembly for Twin Screw Extruders

The preceding embodiments have illustrated the use of die assemblies inaccordance with the invention in the context of single screw extruders.However, the invention is not limited in this respect, and dieassemblies embodying the principles of the invention can be fabricatedfor a use with twin screw extruders.

Referring to FIG. 17, a twin screw extruder 250 is illustrated, with adie assembly 252. The extruder 250 is itself conventional, and includesa tubular barrel terminating in a final head section 254 equipped withan internal, straight sleeve 256. A pair of side-by-side drive shafts258, 260 are located within the extruder barrel and are powered foraxial rotation. The shafts 258, 260 support a pair of juxtaposed,intercalated, axially rotatable, helically flighted screw assemblies,including terminal screw sections 262 and 264. A series of steam locks266 may be mounted on shafts 258, 260 downstream of the screw sections262, 264.

The die assembly 252 includes a tubular section 268 which is secured tothe butt end of terminal extruder head section 254 and communicates withthe tubular extruder barrel. The tubular section 268 is equipped with aspecialized sleeve 270 presenting a smooth inner surface 272 with aprojection surface 274 on one side section thereof, both at a divergenceangle of about 9°. The inlet end 268 a of the tubular section 268 islarger than the output end 268 b thereof.

The assembly 252 also includes a pair of screw sections 276, 278 withintubular section 268. The screw section 276 is mounted on drive shaft 260and has a central shaft 280 with outwardly extending helical Righting282 presenting outermost fighting surfaces 284. The inlet end 276 a ofthe screw section 276 is smaller than the outlet end 276 b thereof. Thescrew section 278 is mounted on drive shaft 258 and includes a centralshaft 286 with outwardly extending helically fighting 288 presentingouter Righting surfaces 290. The inlet end 278 a is larger than theoutlet 278 b. The axial length of screw section 278 is less than that ofmating screw section 276. The fighting 282 of screw section 276 isintercalated with the Righting 288 of screw section 278, i.e., theflighting 282 extends past the outer surfaces 290 of the fighting 288,and vice versa. It will be seen that the outer fighting surfaces 284,290, and the sleeve surface 272 cooperatively define a flow path 291which diverges along the length of the die assembly 252.

The die assembly 252 includes a final die 292 having a central mountingstub shaft 294 and a series of restricted die openings 296 incommunication with flow path 291. That is, material passing through thedie assembly 252 diverges along the region between the fighting 282 ofscrew section 276 and surface 272, and also during passage between theintercalated fighting of screw sections 276, 278. Finally, materialpassing along the region between fighting 288 of screw section 278 andsurfaces 272, 274 is likewise ultimately diverged outwardly by the outerend of projection surface 274. Accordingly, the net effect of dieassembly 252 is to provide a desirable divergence of material flow inorder to increase throughput through the extruder 250.

Embodiment of FIG. 19 Use of Back Pressure Valve Assembly BetweenExtruder and Die Assembly

FIG. 19 illustrates a single screw extruder having a final head 24,including a tubular section 44 and cooperating screw section 30, all asillustrated and described with reference to FIG. 4. Inasmuch as thesections 24 and 44 and the screw sections 30 and 56, are identical withthe FIG. 4 embodiment, like reference numerals have been used in FIG. 19and no further discussion components is required. Furthermore, the FIG.19 embodiment makes use of a die assembly 90 as described in theembodiment of FIGS. 5-12. Here again, because the components of the dieassembly 90 are identical with those of FIGS. 5-12, like referencenumerals have been used.

The FIG. 19 embodiment differs from that of the earlier embodiments byuse of a back pressure valve assembly 300 interposed between the tubularsection 44 and die 90. Specifically, the assembly 300 is of the typefully illustrated and described in U.S. Pat. No. 6,773,739. Thedisclosure of that patent relating to the back pressure valve assembly14 thereof is incorporated by reference herein.

The back pressure valve assembly 300 includes three interconnectedcomponents, namely inlet transition 302, valve unit 304, and outlettransition 306. These components are aligned end-to-end andcooperatively define a passageway 308 throughout the entirety of theassembly 300.

In more detail, the transition 302 is secured to the end of tubularsection 44 and has a converging opening 310. The valve unit 304 includesan upright tubular segment 312 generally transverse to the longitudinalaxis of passageway 308 and having a laterally extending opening 314; theupper and lower ends of the segment 312 include sealing rings 316 and318. An elongated valve member 320 is situated and vertically reciprocalwithin segment 312. The valve member 320 includes a laterally extendingthrough opening 322 as well as a product diversion passageway or channel324 including an inlet opening 326 and outlet 328. The valve member 320is selectively moveable within segment 312 by means of piston andcylinder assembly 330. In particular, the assembly 330 is supported bymounting block 332 secured to segment 312. The assembly 330 includes areciprocal piston rod 334 secured to the upper end of valve member 320.The outlet transition 306 is secured to the outer face of segment 312and has a diverging opening 336. The die assembly 90 is secured to theoutlet face of transition 306 by means of bolts 338.

In operation, the back pressure valve assembly 300 may be operated tovary the pressure and cook conditions developed in the overallextruder/back pressure valve/die assembly. Specifically, the valvemember 320 is shifted downwardly until opening 322 comes into registrywith passageway 308. During extrusion, the effective cross-sectionalarea presented by the passageway 308 may be adjusted through appropriateoperation of piston and cylinder assembly 330. Of course, the overallextruder and terminal sections 44 and 56 operate as previouslydescribed. Similarly, the die assembly 90 also operates as previouslydiscussed.

Embodiments of FIGS. 20-21 Twin Screw Extruder with Low and High DensityProduct Die Assemblies

Turning now to FIGS. 20 and 21, a twin screw extruder assembly 350 isillustrated. In general, the assembly 350 is conventional and includes amultiple-section tubular barrel with a terminal head 352. The headincludes an internal, smooth sleeve 354 defining an elongated bore 356generally of figure eight configuration. Internally, the assembly 350has a pair of elongated, laterally spaced apart, splined shafts 358 and359. The shafts 358, 359 are powered by a motor and drive assembly (notshown) and support a series of elongated, helically flighted,intercalated screw sections generally referred to by the numeral 360along the lengths thereof.

Referring particularly to FIG. 20, a die assembly 362 is depicted and isoperatively connected to the terminal head section 352. The die assembly362 includes a tubular section 364 secured to head 352 by bolts (notshown). Internally, the section 364 includes a specialized sleeve 366presenting a smooth internal surface 368 which diverges outwardlyrelative to the longitudinal axis of section 364 at an angle of 5°. Thesleeve 366 also includes a helically ribbed internal surface 370 whichdiverges outwardly from the longitudinal axis of section 364 at an angleof 10°. The surface 370 is essentially a continuation of surface 368,save for a transition surface 372 which provides a screw-receivingregion 373.

It will be observed that the shaft 358 has a smooth, non-splined,relatively long extension 358 a within sleeve 366, whereas shaft 359includes a shorter, smooth, non-splined extension 359 a. The extension359 a supports a short, converging screw section 374 within region 373.The extension 358 a supports a diverging screw section 376 which extendssubstantially the entire length of the section 364. The helical fightingof screw sections 374 and 376 are intercalated along the length of thesection 374, with the remainder of screw section 376 extending along theribbed surface 370 of sleeve 366. It will be seen that the screw section376 diverges in conformance with the surfaces 368 and 370, i.e., theportion of screw section 376 adjacent surface 368 diverges at an angleof 5°, whereas the remainder of the screw section diverges at an angleof 10°. Thus, it will be seen that cooperating screw sections 374 and376, together with sleeve 366 cooperatively define a generallyfrustoconical, diverging flow path 377 along the length of the section364.

In the embodiment of FIG. 20, a die assembly 90 as describe withreference to FIGS. 5-12, is secured to the outer end of tubular section364 by means of bolts 378. Accordingly, the reference numerals used inthe discussion of FIGS. 5-12 are likewise used to identify the dieassembly components of this embodiment. In operation, material travelingalong diverging flow path 377 passes into and through the die assembly90 as previously described.

FIG. 21 includes a die assembly 380 which is identical with die assembly362 except for the use of a different type of final die 382. Therefore,like reference numerals are used for the identical components of the dieassemblies 362 and 380. The final die 382 includes a die plate 384having a series of through-openings 386 arranged in a circular patternand in proximity to the end of frustoconical flow path 377. The dieplate 384 is secured to a mounting ring 388 affixed to the butt end oftubular section 366 and includes an internal, central block 390 having athreaded bore 392. A floating knife unit 394 is secured to die plate 384and has a rotatable knife blade set 396. The set 396 is supported by arotatable housing 398 which is supported on bearings 400, enabling theknife blade set to “float” during operation.

In the operation of the FIG. 21 embodiment, material passing along flowpath 377 is extruded through the openings 386 with a pressure dropacross these openings. The knife unit 394 serves to sever the extrudatepassing through the openings 386 to size the product as desired.

EXAMPLES

The following examples set forth preferred apparatus and methods inaccordance with the invention. It is to be understood, however, thatthese examples are provided by way of illustration only, and nothingtherein should be taken as a limitation upon the overall scope of theinvention.

Example 1

In this Example, micro-aquatic floating fish feeds were produced using abasic recipe including 50% by weight soybean meal, 22% by weight fishmeal, 25% by weight wheat flour, 1% by weight calcium carbonate and 2%by weight salt.

The extrusion equipment included a Model 16 DDC Wenger preconditioner,and a 5-head Model X165 Wenger single screw extruder equipped with thedie assembly of FIGS. 1-4. The die plate had a total of 504 1.8 mmdiameter die holes.

Two separate runs were conducted using this recipe and equipment. In run1, 1% of Menhaden fish oil was added to the material in thepreconditioner. In run 2, there was no oil addition.

The following Table 1 sets forth the results from this series of tests.

TABLE 1 RUN NUMBER 1 2 DRY RECIPE INFORMATION Density (kg/m³) 585 585Feed Rate (kg/hr) 3000 5000 Feed Screw Speed (RPM) 71 — PRECONDITIONINGINFORMATION Preconditioner Speed (RPM) 250 250 Steam Flow toPreconditioner (kg/hr) 240 350 Water Flow to Preconditioner (kg/hr) 178300 Preconditioner Discharge Temp (° C.) 86 61 EXTRUSION INFORMATIONExtruder Shaft Speed (RPM) 540 540 Extruder Motor Load (%) 39 56 SteamFlow to Extruder (kg/hr) 150 200 Water Flow to Extruder (kg/hr) 58 500¹Control/Temperature First Head (° C.) 50/48 50/48 ¹Control/TemperatureSecond Head (° C.) 50/58 50/45 ¹Control/Temperature Third Head (° C.)70/71 70/68 ¹Control/Temperature Fourth Head (° C.) 90/86 90/88¹Control/Temperature Fifth Head (° C.) 80/83 80/81 Specific MechanicalEnergy (SME) (kWhr/T) 24.7 — ¹Temperature control involved injection ofcold water or steam into the external jackets of the extruder barrel.

The as-extruded density in runs 1 and 2 was 370 and 430 g/l,respectively. After drying the final product densities were 444 and 488g/l, respectively. The run 1 product was 85% floating and the run 2product was 60% floating. Both products exhibited excellent waterstability.

Example 2

In this Example, micro-aquatic sinking fish feeds were produced usingthe same product recipe and equipment of Example 1, except that the dieassembly of FIGS. 5-12 was employed. The three die inserts each had 1049die holes of 1.5 mm diameter, for a total of 3147 die holes. A total ofsix runs were conducted, and Menhaden fish oil was added to thepreconditioner in runs 1-3 at a level of 3% by weight, and in theremaining runs at a level of 5% by weight.

The following Table 2 sets for the results of these runs.

TABLE 2 RUN NUMBER 1 2 3 4 5 6 DRY RECIPE INFORMATION Density (kg/m³)585 585 585 585 585 585 Feed Rate (kg/hr) 3009 3006 4771 5004 4841 5005Feed Screw Speed (RPM) 70 70 123 133 128 129 PRECONDITIONING INFORMATIONPreconditioner Speed (RPM) 250 250 250 350 250 250 Steam Flow toPreconditioner (kg/hr) 241 239 350 350 250 250 Water Flow toPreconditioner (kg/hr) 175 188 293 — 596 613 Preconditioner DischargeTemp (° C.) 89 92 85 83 76 78 EXTRUSION INFORMATION Extruder Shaft Speed(RPM) 420 300 300 300 420 540 Extruder Motor Load (%) 33 38 61 83 50 45Steam Flow to Extruder (kg/hr) 120 60 101 100 101 100 Water Flow toExtruder (kg/hr) 120 306 495 485 502 512 ¹Control/Temperature First Head(° C.) 50/44 50/50 50/50 50/54 50/51 50/47 ¹Control/Temperature SecondHead (° C.) 50/50 50/51 50/62 50/47 50/54 50/47 ¹Control/TemperatureThird Head (° C.) 70/55 70/74 70/74 70/64 70/70 70/61¹Control/Temperature Fourth Head (° C.) 90/60 90/90 90/89 90/90 90/8590/87 ¹Control/Temperature Fifth Head (° C.) 80/57 80/83 80/78 80/8080/78 80/83 Specific Mechanical Energy (SME) (kWhr/T) 20.3 16.7 18.521.6 20.9 15.9 FINAL PRODUCT INFORMATION As-Extruded Density (g/l) 630640 630 618 630 640 Dry Density (g/l) 600 642 664 648 656 646 PercentFloating (0% floating/seconds)  0/30  0/20  0/15  0/25  0/20  0/30 WaterStability (Hrs) 10 10 8 3 3.5 3.5 ¹Temperature control involvedinjection of cold water or steam into the external jackets of theextruder barrel.

The products from Examples 1 and 2 had densities and water stabilitieswithin industry standards for floating and sinking fish feeds. In termsof production rates, typical feed production rates for micro-aquaticfeeds using a Wenger Model X165 extruder are in the range of 1-1.5tons/hr. However, using the die assemblies of the invention, productionrates of 5 tons/hr. or greater were achieved. These levels wereachieved, which were the maximum rates possible using the feederequipment associated with the extrusion systems. Greater throughputscould be achieved with upgraded feeder equipment.

1. A method of extruding a food material from an elongated extruderbarrel presenting a longitudinal axis, comprising the steps of: causingsaid food material to pass from said extruder barrel and along aprogressively diverging, generally frustoconical path of travel, saidpath of travel being defined between an elongated, tubular sectionhaving an internal bore presenting a longitudinal axis, an inlet end andan outlet end, said bore progressively diverging in a direction fromsaid inlet end toward said outlet end, and an elongated, axiallyrotatable, food material-conveying screw section within the tubularsection, said screw section having an axial length, a smaller diameterinlet end proximal to said tubular section inlet end, an outer diameteroutlet end proximal to said tubular section outlet end, said screwsection including an elongated shaft with outwardly extending helicalfighting presenting fighting outer surfaces along the length of thescrew section, said fighting outer surfaces progressively diverging in adirection from said screw section inlet end to said screw section outletend; thereafter dividing the food material from said frustoconical pathof travel, and passing the food material into and through a plurality ofelongated, separate, structurally distinct tubular extensions definingindividual paths of travel, each of said extensions oriented at adiverging angle relative to said barrel longitudinal axis; and forcingsaid food material traveling through said extensions along saidindividual paths of travel through a plurality of restricted orificefood extrusion die openings, including the step of creating a pressuredrop as said food material is passed through said die openings.
 2. Themethod of claim 1, said extensions each independently oriented at anangle of from about 2-12° relative to said barrel longitudinal axis. 3.The method of claim 2, said angle being from about 4-10°.
 4. The methodof claim 1, said frustoconical path of travel having an angle ofdivergence of from about 1-11° relative to said barrel longitudinalaxis.
 5. The method of claim 1, including the step of forcing said foodmaterial through said restricted die openings without recombining thematerial passing through along said individual paths of travel.
 6. Themethod of claim 1, including the step of heating said material prior tosaid material-dividing step.
 7. The method of claim 6, including thestep of heating said material to a temperature of from about 100-150° C.8. The method of claim 1, said method serving to achieve a level of cookof the food material of at least about 75%.
 9. The method of claim 1,said food material including one or more grains selected from the groupconsisting of corn, wheat, oat, milo, and soy.